The present invention relates to a device and a method for high-rate etching a substrate, in particular, in an anisotropic manner, using a plasma etching system, it being possible to achieve periodically varying plasma powers of up to 5000 watts as well as to a device and a method for igniting a plasma and to adjusting upward or pulsing the plasma power according to the species defined in the independent claims.
German Patent 42 41 045 C1 describes a method for anisotropically etching silicon using high etching rates and high mask selectivity, a high-density plasma source having preferably inductive high-frequency excitation being used for liberating fluor radicals from a fluor-delivering etching gas and (CF2)x-radicals from a passivating gas delivering Teflon-forming monomers. In the process, etching gas and passivating gas are used alternatingly, a side wall polymer film being built up on the side walls of already etched patterns during the passivation steps or polymerization steps, the side wall polymer film, during the per se isotropic etching steps, each time being partially removed again with the assistance of ions and, at the same time, the silicon pattern ground being etched by fluor radicals. This process requires a high-density plasma source which also generates a relatively high density of ions (1010-1011 cmxe2x88x923) of low energy.
An increase in the etching rate, which is required for many applications, is generally to be expected when the high-frequency power coupled into the plasma is increased.
In methods as described in German Patent 42 41 045 C1, however, this is surprisingly not the case. Instead, it is observed that the etching rate in silicon increases only slightly in response to increasing the power of the plasma source while, at the same time, unwanted profile deviations, in particular, in the upper third of the produced trenches, greatly increase, resulting in profile indentations or undercuts of the mask edge.
These effects, on one hand, originate from unwanted capacitive interferences from regions of the inductive plasma source, which carry very high high-frequency voltages. When working with higher powers and voltages, these unwanted interference effects are naturally higher as well.
In so far as the plasma source itself is affected, the mentioned effects can be rectified at least to a great extent by advanced feed concepts of the plasma source and, for example, by using a special aperture as is described in German Patent 197 34 278 C1. However, those profile deteriorations which are due to the process and therefore have to be tackled from the process side remain.
While in simple plasma patterning processes, an increase in the plasma power, because of the resulting increased production of ions and etching species, gives rise to the desired increase in the etching rate, in the case of the method according to German Patent 42 41 045 C1, the deposition steps must also be taken into account in addition to the etching steps. In this context, an increase in the plasma power during the etching steps not only results in the desired increased production of etching species and ions but also changes the deposition steps in a characteristic manner.
A very important aspect of German Patent 42 41 045 C1 is the side wall film transport mechanism which, during the per se isotropic etching steps, assures that the side wall protective film is also moved into the depth of the trench during further etching and that it can provide local edge protection already there. However, during the deposition or polymerization steps themselves, such a transport mechanism is only desired within certain limits. Thus, the intention is, in particular, to prevent that excessive side wall polymer is driven down into the trenches already during the deposition cycles and that it is then lacking above, i.e., the side wall film gets too thin there.
If the plasma power is increased during the etching and deposition steps, for example, in the case of a process according to German Patent 42 41 045 C1, an increased polymer transport from the side wall into the depth of the trenches takes place, per se unintentionally, also during the deposition steps in competition to the coating of the side walls since, above a certain plasma power, the deposition rate can no longer be substantially increased but, instead, ions are increasingly produced which impinge on the substrate to be etched.
Due to the plasma potential which lies somewhat above the substrate potential even without an additionally applied substrate electrode voltage, this increasing ion flux toward the substrate results in that an increasing part of the deposited film material is pushed into the depth of the trenches and to the etching ground already during the deposition steps. In particular, the plasma has a plasma potential of several Volts up to several 10 Volts with respect to grounded surfaces and, consequently, also with respect to a substrate on the substrate electrode, which is tantamount to a corresponding ion acceleration toward the wafer. Therefore, an increased ion density also signifies an increased ion action upon the substrate surface and, in particular, upon the trench side walls although, explicitly, no ion acceleration voltage is applied to the substrates.
As a result of the explained polymer removal and carrying over into the depth of the trenches already during the deposition steps in the case of very high plasma powers, the polymer material needed for the side wall protection in the subsequent etching steps is finally lacking in the upper parts of the etched trenches when working with high plasma powers, which manifests itself in the mentioned profile deviations more or less in the upper third of the trench profile. At the same time, the polymer material transported to the etching ground in excess also disturbs the etch removal during the subsequent etching steps and, on the whole, leads to the observed saturation of the etching rate in spite of the further power increase in the source. A further effect in this connection is the xe2x80x9chardeningxe2x80x9d of the deposited polymer material when working with very high power densities, i.e., an increased carbon content in polymers compacted in this manner, which makes the subsequent polymer removal more difficult and, consequently, reduces the etching rates.
Therefore, it is an object of the present invention to overcome a saturation of the etching rate in spite of a higher high-frequency power provided by the plasma source, thus drastically increasing the etching rate. A further object of the present invention is to enable the ignition and the coupling in of very high high-frequency powers into a, in particular, inductive plasma source in a stable manner.
The devices according to the present invention and the methods according to the present invention having the characterizing features of the independent claims have the advantage over the background art that they allow the high-frequency power applied to a plasma source to be periodically changed so that, for example, alternating deposition or polymerization and etching steps can be carried out very advantageously using high-frequency powers of different magnitude. In this context, a higher high-frequency power is in each instance very advantageously applied to the plasma source during the etching steps than during the deposition steps.
Moreover, using the device for etching according to the present invention and the method for anisotropically etching a substrate according to the present invention, considerably higher etching rates can be achieved than with known etching methods and etching devices. In this context, the difficulty existing in known methods heretofore that, in spite of a continuous increase in the plasma power, a saturation of the etching rate occurs in anisotropic etching methods where deposition steps and etching steps are used alternatingly.
Furthermore, it is very advantageous that the device according to the present invention and the method for igniting and adjusting upward a plasma carried out therewith, for the first time, allow very high high-frequency powers to be coupled into a, in particular, inductive plasma source in a stable manner.
Advantageous embodiments of the present invention are derived from the measures specified in the subclaims.
Thus, the method according to the present invention allows the method according to German Patent 42 41 045 C1 to be considerably improved in a very advantageous manner by applying a low plasma power during the deposition steps and by applying a very high plasma power during the etching steps, extremely high etching rates being attained, for example, in silicon while retaining the advantages known from German Patent 42 41 045 C1. In the etching method according to the present invention, in particular, the deposition steps very advantageously remain nearly unchanged. Moreover, the etching steps are advantageously carried out using very high plasma powers of up to 5000 watts at preferably increased SF6/O2 flow and preferably increased process pressure.
Besides, the uniformity of the etching process is significantly improved by switching back the high-frequency power, according to the present invention, during the polymerization steps so that the substrate center and the substrate edge have nearly identical etching rates. This is true, in particular, if the method for high-rate etching according to the present invention is combined with an aperture device in the plasma etching system as is known from German Patent 197 34 278. A very particularly advantageous variant of the method according to the present invention with regard to the uniformity of the etching over a wafer results if a plasma etching system as is known, for example, from German Patent 197 34 278 is further operated using a symmetrically fed plasma source as is proposed in German Application 199 00 179.
Moreover, the device for etching a substrate according to the present invention allows very high high-frequency powers of up to 5000 watts to be coupled into, in particular, inductive plasma sources in a stable manner. To this end, provision is advantageously made for a second means, in particular, for an automated impedance transformer which is controlled in a manner corresponding to the variation in the high-frequency power of the plasma source. Besides, the speed-adapted variation in power of the plasma source or of the high-frequency generator feeding the plasma source, respectively, is at the same time achieved in an advantageous manner via a ramp generator.
In this context, controlling the plasma power using a high-frequency generator and a ramp generator being in communication therewith as well as an impedance transformer for adapting the impedance, in particular, in a continuous and automated manner, is very advantageously suitable both for igniting and for adjusting upward a plasma up to highest power values and for alternating the power parameters at the plasma source between etching and deposition steps according to the present invention.
The increased formation of etching species through a higher plasma power can further be promoted in an advantageous manner by increasing the flow of the fluor-delivering etching gas, for example, SF6 simultaneously with the increase in power. In this case, to prevent sulfur depositions in the exhaust area of the etching system, the oxygen content is to be advantageously adjusted correspondingly. A further expedient way of increasing the production of fluor radicals concurrently with the power increase during the etching steps is increasing the process pressure. In this manner, fluor radicals are increasingly produced in the etching plasma in place of additional ions, thus increasing the ratio of the number of fluor radicals to the ion density. The exceeding of a certain ion density in the case of very high plasma powers is a disadvantage.
Besides, the power is advantageously not increased to, for example, more than 1500 watts during the deposition or polymerization steps. Since the deposition rate on the substrate suffices already when working with a relatively small power of 400 watts to 800 watts, an increase in power of the plasma source during the deposition steps combined with otherwise unchanged plasma etching parameters would, in any case, yield only few additional deposition species or would excessively compact the deposited polymer and lead to a carbon concentration in the polymer. Moreover, by maintaining the original small power of up to 1500 watts during the deposition process, it is, at the same time, advantageously avoided that the ion density and, consequently, the ion action upon the substrate are increased during the deposition steps. Because of this, the explained detrimental consequences of an increased ion density during the deposition steps do no occur.